1. Field of the Invention
This invention relates generally to self-tuning variable impedance circuits, for example as may be used for impedance matching of power amplifiers.
2. Description of the Related Art
Power amplifiers are a commonly used component in many types of systems. For example, the recent increase in wireless systems (e.g., the cellular phone network) has resulted in a corresponding increase in wireless handsets. Within these handsets, power amplifiers are commonly used to boost signal powers and can account for a significant fraction of the overall power used by the handset. As a result, it is desirable to reduce the amount of power consumed by power amplifiers, particularly in mobile devices where power consumption limits characteristics such as battery life, talk time and standby time. However, if a power amplifier is required to boost signal power by a certain amount, the amplifier's consumption cannot be straightforwardly reduced simply by feeding less power to the amplifier. Rather, the efficiency of the power amplifier need be improved, thus achieving the same signal boost with less consumed power.
One way of increasing the efficiency of a power amplifier is to match the load impedance to the power amplifier. However, the optimal matching impedance typically varies as a function of the signal power. For a higher power output from the amplifier, a lower impedance typically results in higher efficiency. In contrast, for a lower power output, a higher impedance typically results in higher efficiency.
In one common approach, the load impedance is designed for optimal efficiency at the highest power output for the power amplifier. However, this results in less than optimal efficiency at lower powers. In another approach, the power output of the amplifier is detected and a control circuit actively changes the load impedance to match. However, this approach requires a control algorithm, which adds complexity. It also requires additional control circuitry, which adds to the overall power consumption. In another approach, two different load impedances are used, one for higher output powers and one for lower output powers. The power amplifier is switched between the two, depending on the actual output power. This approach has the drawbacks of both of the previous approaches. It requires additional circuitry to control the switching. It also is not optimal for many output powers since, for example, neither load impedance will be optimal for intermediate output powers.
Thus, there is a need for an approach that provides a variable impedance circuit, as may be used for impedance matching to power amplifiers. The variable impedance circuit preferably is also inexpensive with low power consumption.